Prime Minister John Key won't rule out changing the inquiry into Judith Collins' actions into a royal commission after NZ First leader Winston Peters made it a bottom line for his support.

But Key said today the difference between a royal commission and what he had established was largely semantic. He would wait until after the election to say if it would be changed.

"Winston Peters has a lot of bottom lines; this is just another one," Key said.

"As I've said before, I'm not going to get into bottom lines before an election. We'll go and talk to political parties afterwards."

That should wait until the makeup of any government was known after the election, he said.

Peters told Radio New Zealand that NZ First would not work with, or offer confidence and supply to, a party that did not agree to such a royal commission.

The inquiry should look into the allegations raised by Hager's book, which outlined the involvement of senior members of the National Party in attack politics against political opponents, he said.

If other parties were involved in such tactics, they would also be swept in.

NZ First's position was "rock-solid", and it would not deviate from it, Peters said.

Speaking to reporters after a speech to the Canterbury Employers Chamber of Commerce, Key said the powers of the inquiry were identical to those of a royal commission.

The only difference was who appointed its personnel - the governor-general in the case of a royal commission or the prime minister in this case - and who it reported to.

"In principle it's the same."

The terms of reference were always flexible enough to change, but asked if he would broaden them in response to Peters' wishes, Key said: "My view it that this is a very specific allegation and it needs to be answered and we need a commission if inquiry to get to the bottom of that and I'm very, very confident that we are going to do thorough, a proper and a professional job.

"We can do that with a commission of inquiry and any other discussions are for another day."

It would likely either be led by a retired high court Judge or a Queen's Counsel.

Things could always change, and Key referred to the 2005 election when Peters said he would not take the baubles of office.

"You'll probably be able to find some footage of him in 2006 driving around in a government limousine."

But Key would not say if he would refuse to go into government rather than extend the inquiry.

It was possible that because the hacker of Whale Oil blogger Cameron Slater was a "criminal hacker" some things he had released were forgeries as former justice minister Collins had suggested.

But, Key said, he just didn't know.

"I wouldn't want to pass judgment either way, it's ultimately for the illegal criminal to demonstrate that they're ... real or not."

Political polls have had NZ First in the role of kingmaker, where neither a Labour-Greens alliance, nor National and its coalition partners, would be able to form a government without it.

Peters said Key should not be ordering any inquiry this close to the election. With voters going to the polls in less than three weeks, Key's constitutional role was that of a caretaker. The decision should be left to the government elected after September 20.